To put it in a way that is easier to picture, Lord Falconer has lost weight equivalent to 211 apples or 86 cans of Diet Coke, the food and drink that he says partly explains his dramatically reduced silhouette.

But even those figures are hard to picture, so much has the former political heavyweight jettisoned. So try this: before he started his new regime two years ago, the Labour peer carried extra weight equal to two car wheels or half a Tom Daley.

Falconer’s new, 11st 5lb (72kg) look, which he explained on Sunday, partly to reassure colleagues who thought that he had been ill, makes the QC and former Lord Chancellor, 62, one of the biggest losers in the unofficial (non-existent) Westminster Fat Club. Its captain: Nigel Lawson, whom Falconer revealed as an inspiration. The former Chancellor lost the same amount in half the time by ditching dairy, sugar and booze, and in 1996 published a diet book.

The two shadows of their former selves reveal the challenges to one’s waistline that wait in Westminster, a machine that is traditionally lubricated by alcohol, gravy and custard. While many MPs yo-yo or prefer to reserve their generous spaces on the benches (Eric Pickles and John Prescott spring slowly to mind) others increasingly feel compelled by their doctors or merciless media attention to shed loads of weight.

Ann Widdecombe, who lost two and a half stone, notably by appearing on ITV’s Celebrity Fit Club in 2002, recalls the perils of her 23 years in Parliament, most of them on the Tory front bench. “Before I arrived, all I had was a bite for lunch and then, being young and single, I didn’t cook a huge meal for dinner,” she says. “When I went into Westminster, almost every day I had an official lunch or dinner, and then fetes with cream teas at weekends. My waistline grew rounder until I left when it disappeared.”

Widdecombe, who says she lost weight even in summer recess, reveals no secret method. “If I wrote a diet book, it would be two pages,” she says. “Eat less, exercise more, that’s it. For Lent, I give up everything except water to drink – no alcohol, tea, or fizzy drinks – and do I lose a pound? No, I do not.”

Ex-members of the Westminster Fat Club

Ex-members of the Westminster Fat Club

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Lord Falconer in 2005

Getty

2/8
...and 2014

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Nigel Lawson in 1985

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...and 2006

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Ann Widdecombe in 1999

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...and 2014

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George Osborne in 2013

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...and 2014

But MPs can be as faddy as any modern slimmer. Lord Falconer, who also now runs most days, started following the popular 5:2 diet, also favoured by the newly-svelte George Osborne, as well as House bruiser Ed Balls. Michael Gove, who got podgy following marriage to the Lib Dems in 2010, whipped himself into shape at an Austrian “fat farm” last year. The former Education Secretary reportedly paid £2,500 to eat soup and stale bread at the Mayr Clinic, but passed on the colonic irrigation. He went on to lose two stone, prompting speculation that it was part of a strategy to challenge David Cameron for the Conservative Party leadership.

In the age of constant media scrutiny and the cartoonist’s spear-like pen, it’s hard to imagine a seriously overweight party leader. Thatcher was a weight freak, Tony Blair was so athletic that his “butt” won the private admiration of Wendi Deng, Rupert Murdoch’s ex-wife, and Cameron has made a show of cycling and jogging. Pity those who don’t have such discipline. Nicholas Soames, the Tory MP and grandson of Winston Churchill, has been nicknamed variously Bunter, Fatty, Creepy Crawley, and Crawley Food Mountain (after his former constituency, Crawley in Sussex). An anonymous woman once described making love to Soames as “like having a wardrobe fall on top of you with the key sticking out”. In the House, an MP interrupted an attack that the MP was making on the Millennium Dome to suggest: “You could have an exhibition inside your own underpants.”

Even politicians who aren’t fat can become so on paper. Dave Brown, The Independent’s cartoonist, recalls drawing Gordon Brown while he was Prime Minister. “He had a big, heavy-looking face and was good to draw as a colossal heavyweight, especially if I was comparing him to Blair,” he says. “He did once complain that I drew him too fat, which was the worst thing to say to a cartoonist because, of course, you then make him even fatter.”

Dave Brown is bound by his profession to be mean but says he only uses weight to make a point. He depicted Eric Pickles, perhaps the physical heir to Soames, as a giant sandbag after the Conservative minister had been accused of “grandstanding” by criticising the Environment Agency’s response to the Somerset floods last February. The cartoonist prefers politicians to hang on to their distinguishing features, but Lawson’s change offered some compensation. “He had been so fat that when he lost weight he looked like an empty bag,” Brown says. “There was so much loose flesh I could still get a really good caricature.”

There is help inside Westminster for MPs who feel the pressure to slim. The parliamentary gym is busy (athletic moles report that Jack Straw and Peter Bone are keen members) and the All-Party Parliamentary Weight Watchers Group meets most weeks, attended by more than 40 MPs and other staff, as well as two advisers from Weight Watchers. “We all get weighed and a record is kept of increases and losses,” says the group’s chair, the Labour MP Adrian Bailey, after eating a Morrison’s chicken sandwich for lunch. “But I think membership has dropped since 2010 because the new intake was slimmer and fitter than before.”

The 68-year-old says that he has lost a stone by swimming three miles a week, using the stairs and “turning down all the free wine here” but reveals, perhaps unsurprisingly, that some members of the group have found ways to fiddle the records. “An MP I won’t name who was a big man in all directions had a quite spectacular weight loss,” he says. “But then he confided to me that he had been standing near enough to a table during a weigh-in that he could rest his stomach on it.”